1. Field of the Invention
The present invention relates to a polishing apparatus for polishing a plate-like workpiece such as a semiconductor wafer or a glass substrate.
2. Description of the Related Art
Recent rapid progress in semiconductor device integration demands smaller and smaller wiring patterns or interconnections and also narrower spaces between interconnections which connect active areas. One of the processes available for forming such interconnection is photolithography. Although the photolithographic process can form interconnections that are at most 0.5 xcexcm wide, it requires that surfaces of semiconductor wafers on which pattern images are to be focused by a stepper be as flat as possible because the depth of focus of the optical system is relatively small. It is therefore necessary to planarize the surfaces of the semiconductor wafers for photolithography. One customary way of planarizing the surfaces of the semiconductor wafers is to polish them with a polishing apparatus.
FIG. 17 of the accompanying drawings shows a main part of a conventional polishing apparatus. The polishing apparatus comprises a rotating polishing table 100 with a polishing cloth 102 made of urethane or the like attached to an upper surface thereof, a top ring (workpiece holder) 104 for holding a semiconductor wafer W which is a workpiece to be polished and pressing the semiconductor wafer W against the polishing table 100 while the top ring 104 is rotated, and a polishing liquid supply nozzle 106 for supplying a polishing liquid Q to the polishing cloth 102. The top ring 104 is connected to a top ring shaft 110 through a spherical bearing 108 so that the top ring 104 is tiltable with respect to the top ring shaft 110. The top ring 104 is provided with an elastic pad 112 made of polyurethane or the like on its lower surface, and the semiconductor W is held by the top ring 104 in contact with the elastic pad 112. The top ring 104 also has a cylindrical guide ring 114 mounted on a lower outer circumferential edge thereof for retaining the semiconductor wafer W on the lower surface of the top ring 104.
In operation, the semiconductor wafer W is held against the lower surface of the elastic pad 112, and pressed against the polishing cloth 102 on the polishing table 100 by the top ring 104. The polishing table 100 and the top ring 104 are rotated to move the polishing cloth 102 and the semiconductor wafer W relatively to each other. At this time, the polishing liquid Q is supplied onto the polishing cloth 102 from the polishing liquid supply nozzle 106. The polishing liquid Q comprises a chemical solution such as an alkali solution containing abrasive particles suspended therein. The semiconductor wafer W is polished by a composite action comprising a chemical polishing action of the chemical solution and a mechanical polishing action of the abrasive particles. This polishing is called chemical mechanical polishing.
In the chemical mechanical polishing (CMP) apparatus using the polishing cloth 102, since the polishing cloth 102 is made of material having elasticity, irregularities of a polished surface of the semiconductor wafer remain, and the surface of the semiconductor wafer cannot be sufficiently planarized. Therefore, the conventional CMP apparatus cannot cope with a demand for a higher degree of planarization of the semiconductor wafer.
To be more specific, a device pattern on the upper surface of the semiconductor wafer W has various irregularities having various dimensions and steps. When the semiconductor wafer W having step-like irregularities is planarized by the polishing cloth 102 having elasticity, not only raised regions but also depressed regions are polished, and hence irregularities of the polished surface of the semiconductor wafer are difficult to be eliminated, with the result that a high degree of flatness of the polished surface cannot be obtained.
Further, the surface of the polishing cloth 102 tends to have irregularities, and hence it is necessary to perform dressing of the surface of the polishing cloth 102 frequently for thereby removing glazing of the surface of the polishing cloth 102.
Furthermore, a considerable proportion of the polishing liquid Q supplied to the polishing cloth 102 is discharged without reaching the surface of the semiconductor wafer to be polished. Consequently, the polishing liquid Q is required to be supplied in a large quantity, and hence an operating cost of the polishing process becomes high because the polishing liquid is expensive and the cost of a process for treating the polishing drain liquid is high.
Therefore, there has been developed a fixed abrasive type polishing apparatus and method in which polishing surface comprising an abrading plate, i.e. a fixed abrasive plate is used, in place of the polishing cloth 102. The abrading plate comprises abrasive particles such as silica particles and a binder for binding the abrasive particles, and is formed into a flat plate. FIG. 18 shows a main part of a conventional polishing apparatus having such abrading plate. The polishing apparatus comprises a polishing table 100 with a polishing tool 120 attached to an upper surface thereof, and liquid supply nozzles 124 connected to a liquid supply device 122 for supplying water or a chemical liquid during polishing. The polishing tool 120 attached to the upper surface of the polishing table 100 comprises a base plate 116 and an abrading plate 118 attached to the surface of the base plate 116. Other structure of the polishing apparatus shown in FIG. 18 is the same as that of the conventional polishing apparatus shown in FIG. 17.
According to the above polishing process, the abrading plate (fixed abrasive) is harder than the polishing cloth and has less elastic deformation than the polishing cloth, and hence only the raised regions on the semiconductor wafer are polished and undulation of the polished surface of the semiconductor wafer is prevented from being formed. Therefore, selective polishing performance of the raised regions on the semiconductor wafer is improved, a degree of flatness of the semiconductor wafer is improved, and an expensive polishing liquid Q is not required to be used.
Further, it is confirmed by the inventors of the present application that in the polishing method using the fixed abrasive, the polished surface of the semiconductor wafer is planarized once to a certain level, and then the polishing rate is extremely lowered to show a self-stop ability of polishing because of the nature of the fixed abrasive. Therefore, the inventors of the present application have proposed to utilize such self-stop ability of polishing for detecting an endpoint of polishing or detecting a thickness of a film formed on the semiconductor wafer W in Japanese patent application Nos. 10-150546 and 10-134432.
Recently, there have been strong demands of the polishing apparatus for polishing semiconductor wafers towards improvement of productivity per an apparatus and improvement of productivity per unit installation area of the apparatus, as in other semiconductor manufacturing apparatuses. However, in the polishing apparatus having a single top ring per a polishing table, the polishing surface on the polishing table is not effectively utilized, and therefore the productivity per unit installation area of the apparatus cannot be improved.
One solution is to provide a polishing apparatus with a plurality of holders each for holding a workpiece to be polished, the holders sharing a common polishing surface. Such a polishing apparatus is referred to as a multihead polishing apparatus. The multihead polishing apparatus is advantageous in that it can simultaneously polish an increased number of workpieces per unit time. However, it is difficult for the multihead polishing apparatus to polish workpieces to a uniform finish because the simultaneously polished workpieces tend to be polished to different levels at the termination of the polishing process.
Inasmuch as a plurality of workpieces are simultaneously polished with the common polishing surface, if the number or the position of workpieces to be polished changes, then the forces applied to the common polishing surface also change, making it difficult to keep the common polishing surface at a desired attitude or posture. As a result, the workpieces cannot be polished as desired.
It is therefore an object of the present invention to provide a polishing apparatus which is capable of increasing the throughput of the workpieces per unit installation area in a clean room requiring an expensive operating cost with keeping uniformity and quality of polished surfaces of the workpieces.
According to an aspect of the present invention, there is provided a polishing apparatus for polishing a surface of a workpiece, comprising: a polishing table having a polishing surface thereon; a plurality of workpiece holders each for holding a workpiece and pressing the workpiece against the polishing surface; and a controller for controlling the workpiece holders individually so that polishing operations of the workpiece holders are controlled independently of each other.
Since a plurality of workpieces can simultaneously be polished on one polishing table, the throughput per unit installation area can greatly be increased. Furthermore, because the process of polishing the workpieces is individually controlled by the controller, the workpieces can uniformly be polished to a desired level without being polished excessively or insufficiently by individually controlling the process of polishing the individual workpiece. Elements that can be controlled for controlling the polishing process include a polishing time, a polishing pressure, a relative sliding speed between the workpiece and the polishing surface, etc.
The controller may control the workpiece holders so as to be individually movable into and out of contact with the polishing surface.
Inasmuch as the workpiece holders can be moved into and out of contact with the polishing surface independently of each other for individually controlling the polishing times of the workpieces based on a suitable parameter, it is possible to cancel out differences between the polishing characteristics of the workpieces to eliminate variations in the workpieces and to polish the workpieces as required.
The motion of the polishing table within the predetermined plane may be a rotary motion as of a turntable or a scroll motion referred to as a circulative translational motion, which may be selected depending on the purpose of the polishing process.
The apparatus may further comprise a detecting device associated with each of the workpiece holders, for detecting condition of the workpiece which is being polished by the workpiece holder.
Various parameters may be detected as heretofore proposed. For example, if the polishing surface is a fixed abrasive surface which causes self-generation of abrasive particles, then its self-stop ability may be used to perform a simple control process by detecting a sliding torque between the workpieces and the polishing table.
The detecting device may detect formation of a liquid film between the workpiece and the polishing surface.
The apparatus may further comprise a transfer device for transferring workpieces to and from the workpiece holders. If the transfer device can hold a workpiece, then the rate of operation increases for replacing workpieces.
The transfer device may transfer one of the workpieces to and from one of the workpiece holders.
Alternatively, the transfer device may transfer the workpieces all together to and from the workpiece holders.
The polishing table may have an unpolishing surface disposed inwardly of the polishing surface. If the polishing table comprises a turntable, then since the unpolishing surface has a weak polishing capability, it may effectively be used to install a structure for supplying and discharging a polishing liquid or a temperature regulating heating medium, for example.
At least one of the workpiece holders may place the workpiece on a center of the polishing table for polishing the workpiece. Therefore, the surface of the polishing table can effectively be utilized for an increased throughput. If the polishing table comprises a turntable, then the center of the polishing table has a weak polishing capability. However, the weak polishing capability poses no problem particularly if the polishing surface is a fixed abrasive surface because its self-stop ability is effective to produce uniformly polished surfaces on the workpieces.
According to still another aspect of the present invention, there is provided a polishing apparatus for polishing a surface of a workpiece, comprising: a polishing table having a polishing surface thereon, the polishing surface being made of a material which causes self-generation of an abrasive; and a plurality of workpiece holders each for holding a workpiece and pressing the workpiece against the polishing surface. Since the polishing surface is a fixed abrasive surface, its self-stop ability can be used when a certain level of flatness is achieved by the workpieces. If a plurality of workpieces having different polishing characteristics are polished for a period of time greater than a certain threshold value, then the levels to which the workpieces are polished converge to a certain value.
According to yet another aspect of the present invention, there is provided a polishing apparatus for polishing a surface of a workpiece, comprising: a polishing table having a polishing surface thereon; a plurality of workpiece holders each for holding a workpiece and pressing the workpiece against the polishing surface; and a noncontact bearing for supporting the polishing table in a noncontact manner while controlling an attitude thereof. Even if the workpiece holders are individually brought into and out of contact with the polishing surface and hence the load on the polishing table is locally changed, the polishing table can be maintained at a constant attitude, allowing the workpieces to be polished well stably.
According to yet another aspect of the present invention, there is provided a polishing apparatus for polishing a surface of a workpiece, comprising: a polishing table having a polishing surface thereon; a plurality of workpiece holders each for holding a workpiece and pressing the workpiece against the polishing surface; and a noncontact bearing for supporting the polishing table in a noncontact manner while controlling an attitude thereof. Even if the workpiece holders are individually brought into and out of contact with the polishing surface and hence the load on the polishing table is locally changed, the polishing table can be maintained at a constant attitude, allowing the workpieces to be polished well stably.
The above and other objects, features, and advantages of the present invention will become apparent from the following description when taken in conjunction with the accompanying drawings which illustrate preferred embodiments of the present invention by way of example.